Related cases include application Ser. No. 07/404,028, filed on Sep. 6, 1989, now abandoned and Ser. No. 07/572,754, filed on Aug. 24, 1990, now U.S. Pat. No. 5,210,155.
1. Field of the Invention
The present invention relates to amino-crosslinkable resin compositions, to solid crosslinked polymer compositions prepared therefrom, and to methods for improving coating properties of films and surface coatings based thereon. It also relates to methods for preparing such compositions.
2. Description of the Related Art
A primary component of crosslinkable coating formulations is a resin which can be natural or synthetic. The resin acts as a polymeric coating binder or polymeric coating vehicle for the coating formulation. In addition, most coatings require a solvent, and the coating may also contain a wide variety of additives. Further, many coatings also contain a crosslinking agent, which after application of the coating vehicle to a substrate, reacts chemically with the resin during a curing stage to produce a film containing a crosslinked network. The crosslinked network is necessary for the production of good film properties. The curing stage can be conducted at ambient conditions ("air-dry system"), or at elevated temperatures ("baked system"). In either case, the solvent is evaporated during the curing stage, resulting in a coating film. A number of properties are important for the coating film, including hardness, flexibility, weather resistance (weatherability), chemical resistance, solvent resistance, corrosion resistance, adhesion to various substrates, and impact resistance. The properties depend on may factors including type, molecular weight, monomer composition, and glass transition temperature (Tg) of the resin; type and amount of the crosslinker; curing conditions; curing catalyst; and additives. Variations of these parameters can be used to create a wide range of differences in film properties to fit requirements for a number of diverse applications. However, it is not always possible to optimize all of the desirable properties simultaneously.
Latex based coatings are environmentally superior because they are based primarily on water as a solvent or carrier. They may be applied as an air dry system and find a wide range of applications mainly as architectural coatings. However, the hardness of these coatings is low since the drying mechanism is based on the coalescence of latex particles rather than on crosslinking mechanisms.
Another type of environmentally preferred coating is based on vegetable or fish oils as binder material. Their drying mechanism is based on crosslinking of the internal double bond structure through oxidation. However, these coatings are also of low hardness.
Coatings having improved hardness may be achieved by using alkyd resins as binder materials. Alkyd resins are hybrids of vegetable oils and polyester resins.
The hardness of these materials may be further increased by reduction of the fatty acid levels used in their synthesis.
Esters (oil free alkyds) can provide better hardness and improved mechanical properties but require the inclusion of crosslinking agents and thermal hardening. Typical agents are amino crosslinking agents. The hardness of polyester based coatings can usually be increased by either changing the monomer composition to increase the Tg of the polymer or by increasing the crosslinking density.
The achievement of increased hardness by increasing polymer Tg gives rise to polymers having increased viscosity which in turn may require the use of larger than desirable quantities of solvent to form solutions suitable for coating processes.
On the other hand, an increase in crosslink density of di or polyhydroxy-containing polymers (made from hydroxy functional monomers) may be achieved by increasing the concentration of the hydroxy functional groups present in the polymer. For example, polyester polymers made by condensing an organic diacid and an excess of diol and containing terminal hydroxy groups and having low molecular weights contain a greater number of terminal hydroxy groups available as crosslinking sites than do the higher molecular weight materials. Thus, an increase in hardness of such resins can be achieved simultaneously with a reduction in viscosity and a reduction of the volatile solvent content of coating and paint formulations.
However, a major drawback in the molecular weight approach for controlling hardness is that at molecular weights below about 1000, an increased amount of volatile low molecular weight fractions are produced. These volatile fractions tend to evaporate when the coating is heated to temperatures to initiate baking and crosslinking of the coating and thereby do not participate in the crosslinking reaction.
Bisphenols are well known materials used in the production of epoxy resins, polycarbonate polymers and polyester resins as well as compositions containing these products.
U.S. Pat. No. 4,124,566 discloses the preparation of polyester resins based on the polyester reaction product of aromatic dicarboxylic acids and diols, including bisphenols, by a two stage reaction wherein an aromatic dicarboxylic acid is first esterified by reaction with an aromatic monohydroxy compound, followed by a second stage reaction of this esterification product with a bisphenol compound or a mixture thereof with an aliphatic diol or dihydroxy benzene. These resins are characterized as having superior thermal stability, transparency and chemical stability. They are of relatively high molecular weight as evidenced by high reduced viscosities in excess of 0.9 for the materials produced.
U.S. Pat. No. 4,028,111 discloses polyester polymers based on an alternating polymer of an aliphatic dicarboxylic acid such as adipic acid and a bisphenol such as bisphenol A prepared using an excess of bisphenol such that the bisphenol groups also end-cap the polyester. The free hydroxy group of the bisphenol end cap is then reacted with a compound having quinonediazide groups to produce a light sensitive polymer.
U.S. Pat. No. 4,281,101 discloses the preparation of relatively high molecular weight polycarbonates comprising reacting a mixture of an aliphatic diol, a carbonic acid bis-aryl ester such as diphenyl carbonate and a diphenyl such as bisphenol A to produce a polycarbonate polymer containing diphenyl carbonate end groups of the diphenyl compound. These polymers may then be used as a precursor for further reaction with preferably aliphatic diols and phosgene to produce thermoplastic aliphatic-aromatic polycarbonate elastomers of high molecular weight. Similar polycarbonates are disclosed in U.S. Pat. Nos. 4,216,298 and 4,297,455.
U.S. Pat. No. 3,787,520 discloses a phenolic hydroxy terminated resin which may be used as a crosslinking agent in the preparation of dry powder paint systems based on crosslinkable copolymers of glycidyl methacrylate and an ethylenically unsaturated compound. The hydroxy terminated resin is prepared by reacting an epoxy compound with a diphenol such as bisphenol A to produce a polyether terminated by the diphenol.
It is also known in the art to prepare phenol terminated liquid elastomers by reacting carboxyl terminated polymers of dienes with diphenols such as bisphenol A such that a phenolic hydroxyl group forms an end group in the polymer chain. These phenol terminated elastomers are subsequently used to cross link epoxy resins to produce an improvement in impact resistance. Examples of such systems are disclosed in U.S. Pat. Nos. 3,770,698 and 3,966,837.
Crosslinked epoxy-based resin compositions having improved hardness are disclosed in U.S. Pat. No. 4,713,137. These compositions comprise, in the uncured state, a mixture of epoxy resin, a polyhydric phenolic crosslinking agent, solvent and an additive such as bisphenol-A. They polyhydric phenolic crosslinking agent may be prepared by separately reacting a hexa-alkyl ether of hexamethylol melamine and a polyhydric phenol such as bisphenol-A.
Other epoxy-based resin compositions containing an aromatic polyhydroxy additive such as bisphenol A are disclosed in U.S. Pat. No. 4,025,578 and Canadian Patent 1,042,581.
All of the above mentioned references describe the reaction of bisphenols with epoxy groups or preliminary incorporation of bisphenols in the synthesis of polycarbonate-polyester resins.